The wet process phosphoric acid manufacturing process generally involves reacting sulfuric acid (H.sub.2 SO.sub.4) with phosphate rock containing varying quantities of fluoride, silica, and other impurities. The reaction is carried out at elevated temperatures in large attack tanks. The slurry produced by the reaction comprises a liquid phase of mostly impure phosphoric acid and a solid phase containing essentially calcium sulfate crystals but also containing unreacted phosphate rock, calcium fluoride, silica, organic matter and many other impurities.
The slurry stream is filtered to produce about a 28-32% P.sub.2 O.sub.5 solution to be sold or further processed into the better grades of phosphoric acid. The filter cake produced during the filtration step contains calcium sulfate crystals and a significant quantity of phosphoric acid and other impurities. This cake is washed with water to recover the remaining phosphoric acid and other soluble impurities. The solution, hereinafter called the filtrate, generally has a phosphoric acid concentration of about 20% and contains many soluble impurities, particularly the fluorine and silica produced during the process.
Most prior art methods merely pump the filtrate back to the attack tank for further reaction with phosphate rock and H.sub.2 SO.sub.4. Other methods have attempted to recover the commercially valuable impurities from the filtrate, particularly FSA. Generally the filtrate is pumped to a stripper where it is mixed with H.sub.2 SO.sub.4. The reaction between H.sub.2 SO.sub.4 and the 20% phosphoric acid in the filtrate releases the gaseous silicon tetrafluoride (SiF.sub.4) which is drawn into an educator type scrubber where water is added to form the FSA containing excess silica. The solution of FSA and silicate are then separated through filtration. The FSA is used for commercial sales.
Recovery of SiF.sub.4 from the stripper is dependent on many factors including the grade of mixing, temperature, flow rates, concentration, types of impurities in the filtrate, and the like. A particularly difficult problem which lowers the amount of SiF.sub.4 recovered is the development of foam on the surface of the stripper reaction solution. In the stripper, the agitation and gases produced due to the mixing of the acid and filtrate form a foam which traps the SiF.sub.4 and lowers the amount of SiF.sub.4 recovered from the acid-filtrate mixture. This lowers the efficiency of the overall process and increases the cost of recovering FSA from the filtrate.
A method is, therefore, needed which can reduce the amount of foam produced by mixing the filtrate and H.sub.2 SO.sub.4 in the stripper and allow more SiF.sub.4 to be recovered from the acid-filtrate mixture. This could increase the efficiency of the process and lower the cost of FSA production.
Defoamers have often been used to suppress foam in the wet process attack tank. For example, U.S. Pat. No. 4,540,511 discloses using a mixture of monocarboxylic acids, a monoalkanolamide, and an alcohol to inhibit foam in the phosphate rock-sulfuric acid medium. U.S. Pat. No. 4,083,936 discloses using phosphate esters of aliphatic alcohols as defoaming agents during the manufacture of phosphoric acid by the acidulation of phosphate rock. U.S. Pat. No. 3,437,437 discloses the use of a hydroxyl amine and fatty acid reaction prior art to control foaming during the production of wet process phosphoric acid. U.S. Pat. No. 4,083,936 discloses using phosphate esters of aliphatic alcohols as anti-foaming compositions during phosphoric acid manufacture. Defoamers have not, however, been used in the filtrate strippers to increase evolution of SiF.sub.4 and therefore FSA production.